In the production of elastomeric polymers, such as isobutylene-based elastomers, the product obtained from the polymerization process is often in the form of an aqueous slurry. The water-slurried elastomer is then “finished” to obtain the final elastomer product by dewatering the elastomer, drying the elastomer, and cooling the dried elastomer so that it may be baled/packaged.
U.S. Pat. No. 5,729,911 discloses a process for continuous dewatering/drying of EP(D)M. The process comprises continuously charging wet EP(D)M crumb into a single screw extruder having particular structural features and conveying the wet crumb through the length of the extruder under conditions sufficient to expel water, increase the pressure across the die plate of the extruder and heat the crumb. The dried crumb thus extruded is characterized in that it contains less than 1% water.
However, processes for finishing EP(D)M elastomers may not work for other types of elastomers. For example, some elastomers, such as isobutylene-based elastomers, are temperature sensitive and are not able to be subjected to extreme temperatures common in EP(D)M dewatering and drying processes without degradation. Additionally, different elastomers have different slurry chemistries and different viscoelasticity which affect the ability to dewater and dry the elastomer. For example, a slurry of butyl elastomer, particularly halobutyl elastomer, has a higher pH than a slurry of EP(D)M. This higher pH makes the butyl elastomer crumb more slippery and thus harder to dewater and dry than EP(D)M crumb. Further elastomer characteristics, such as Mooney viscosity, molecular weight distribution, and the presence or absence of long chain branching, may also affect the ability to de-water and dry the elastomer.
A typical finishing line, such as described in U.S. Pat. No. 5,041,249, consists of a dewatering extruder, an optional intermediate drying extruder, and a dryer. Such finishing lines are limited in the amount of elastomer they can finish per hour. One bottleneck is in the dewatering step. As the dewatering extruder receives the slurry, it removes water imbibed in the crumbs through mechanical squeezing of the crumb in the extruder barrel and some minor water flashing at the die. The dewatering extruder removes a great amount of water from the elastomer crumb and is susceptible to contaminants in the slurry water or on the crumb surface (surfactant, etc.) that can induce slipping in the barrel and/or cause waterlogging wherein the crumb floats and can no longer be conveyed.
Another limitation on the capacity of finishing lines is the amount of water in the elastomer crumb fed to the drying extruder. In the drying extruder the elastomer crumbs are heated up by shear forces and water is flashed off at the drying extruder's die exit. If the moisture level of the elastomer crumbs fed to the drying extruder is too high the crumbs will not be adequately dried. However, if the feed moisture level is too low then the final elastomer crumb product will still be wet due to insufficient water flash at the drying extruder's die. Thus, the narrow operating window in which the moisture level of the elastomer crumb entering the drying extruder must be within acts as another bottleneck to increasing the capacity of finishing lines.
Therefore, there still remains a need for improved processes for dewatering and drying elastomers, including temperature sensitive elastomers, such as isobutylene-based elastomers. In particular, there is a need for a process and apparatus that is able to finish large capacities of isobutylene-based elastomers in a single finishing line.